In interferometry, highly precise surface measurement is obtained for various types of optical components using interference fringes that are generated between light reflected from a reference surface and light from a surface under test. The Fizeau interferometer is one instrument of this type that is advantaged for measurement of various optical surfaces, particularly for spherical or planar surfaces having relatively large diameters.
The schematic block diagram of FIG. 1 shows components of a conventional Fizeau interferometer 10. A laser 12 or other highly coherent light source directs light through a beamsplitter 14 and towards a test surface 20, the sample surface to be measured, as well as toward a reference surface provided within Fizeau optics 18. A collimator 16 and Fizeau optics 18 condition the path of light directed toward the target and reference surfaces and bend this light toward the proper angles for the surface being measured. The optical path of target and reference beams is identical through Fizeau optics 18 and collimator 16. Beamsplitter 14 then redirects the returned light to an interference pattern imaging apparatus 24 for display and analysis.
Fizeau interferometry, using the overall pattern of FIG. 1, is advantaged as a method for the interferometric surface inspection of precise spherical and nearly spherical surfaces because the Fizeau reference surface and the surface under test are within close proximity of each other. This reduces the likelihood of optical path disparities and helps to reduce retracing errors experienced by the light reflecting from the Fizeau reference surface and from the surface under test. Ideally, the interference that occurs during Fizeau interferometry only includes known errors on the Fizeau reference surface and errors on the surface under test, because all other sources of wavefront error are common to both paths.
The schematic diagram of FIG. 2 shows a function of Fizeau optics 18 for accurate surface measurement. Incoming light at L 1 is substantially planar, but must be redirected so that it arrives as spherical light at test surface 20, a spherical surface. Under the desired conditions for interferometry, as shown at enlargements E1 and E2, the light heading toward test surface 20, traced along exemplary rays R1, R2, R6, and R8 in FIG. 2, has a specific angular relationship to a reference surface 22. At any point along reference surface 22, this light that is directed toward the sample surface-under-test, that is, toward test surface 20, exits at a normal to surface 22. Reference surface 22 is also termed the Fizeau reference surface. In addition, when test surface 20 has the proper shape, the returning light, as test light, follows the exact same path and is incident on reference surface 22 at a normal. Reference light L2 that is reflected back from Fizeau reference surface 22 is also returned along the same path as the returning test light.
Fizeau objectives that deviate from perfect sphericity will cause the reference and test optical paths to vary slightly from one another upon return through the optical system, creating propagation errors. The greater the deviation of the wavefront from sphericity the greater the propagation error. Also, errors of larger slope, that is, of higher spatial frequency, will also cause similar propagation errors.
Complex optics designs are typically used to create a nearly perfect spherical wavefront from an incoming planar wavefront and to allow the spherical wavefront to be nearly perfectly normal to the Fizeau reference surface as described with reference to FIG. 2. For example, to bend the light to an appropriate angle θ, as shown in FIG. 2, a number of conventional designs provide Fizeau objectives with as many as 4 or 5 or more lens elements, often where the numerical aperture is 0.5 or greater. Conventional solutions have not provided Fizeau interferometry systems that feature reduced parts count, smaller size, lower weight, and reduced cost, at the same time. At best, conventional solutions typically address one of these factors at the expense of others. For example, the use of one or more diffractive optical elements (DOE) has been proposed as one way to simplify lens design and reduce parts count. However, due to the relatively high cost and complexity of DOE device design itself, little or no cost advantage is obtained using this approach. As is well known to those familiar with optics fabrication, the complex lens optics conventionally used for Fizeau interferometry require considerable expense and skill in manufacturing and assembly. Even the slightest errors in surface quality, thickness, radius, and alignment can have a significant effect on the measurement accuracy of these optical assemblies.
The use of an aspheric lens component is one solution that has been advanced for reducing the number of lens elements in the Fizeau optics. For example, commonly assigned U.S. Pat. No. 5,797,493 entitled “Interferometer with Catadioptric Imaging System Having Expanded Range of Numerical Aperture” to Vankerkhove disclosed the use of one or more aspheric lenses in the optical path of a Fizeau interferometer that has refractive elements and a curved reflective surface for beam direction. Similarly, U.S. Pat. No. 7,342,667 entitled “Method of Processing an Optical Element Using an Interferometer Having an Aspherical Lens That Transforms a First Spherical Beam Type into a Second Spherical Beam Type” to Freimann et al. discloses the use of one or more aspheric lenses in a Fizeau interferometer that uses refractive elements.
Although aspheric lenses are known to offer certain advantages, however, there can be practical hurdles that complicate their deployment or diminish their usefulness in various different applications. The need for precision fabrication, testing, and validation of the aspheric surface is a widely recognized problem to the optics designer and can present complex difficulties that are not easily or inexpensively addressed. In the '667 Freimann et al. patent, for example, a second interferometer apparatus is used in order to characterize or calibrate the aspheric lens for its use in a first interferometer apparatus. Design and use of a special-purpose second interferometer as a test fixture for using an aspheric lens in a first interferometer is a costly solution that adds time and complexity to interferometer manufacture, substantially eroding many of the potential advantages of using an aspheric lens in the first place.
Thus, it can be seen that there would be significant advantages to apparatus and methods for inexpensively testing and using an aspheric lens, thereby reducing parts count, size, and complexity of Fizeau interferometry optics.